1. Field of the Invention
The present invention relates to a method of forming a single crystal of a semiconductor compound and to an apparatus for manufacturing the same.
More particularly, the invention relates to a method or an apparatus of forming a single crystal of a semiconductor compound which is formed by using a seed crystal and a liquid encapsulant and which can easily be removed from a crucible.
2. Description of the Related Art
As a conventional method of forming a single crystal of a semiconductor compound, a vertical Bridgman method or a vertical gradient freeze method, which use a seed crystal and a liquid encapsulant, are known. A liquid encapsulant is a material that is easily changed from solid to liquid and back again by heating and cooling. These methods include a step for cooling a single crystal (or ingot, or boule) of a semiconductor compound in a crucible containing a liquid encapsulant. This cooling continues down to room temperature, after growth of the single having a desired crystal orientation is completed.
After the cooling step, the boule is taken out by holding the crucible upside down, by tapping the bottom of the crucible, or by deforming the crucible slightly. However, a long time is needed for this process, because the liquid encapsulant, which exists on the upper portion of the boule, changes to a solid state and disturbs the boule being removed from the crucible. Another problem is that cracks in the boule are often observed near the portion of the seed crystal, if the boule is forcibly removed from the crucible.
Especially, when the crucible is made of pyrolytic BN (pyrolytic boron nitride), the crucible is expected to be reused many times, so the boule has to be carefully removed so as to avoid damage to the crucible.
Two methods are known to solve such problems. A first known method is to dissolve the solid encapsulant by pouring methyl alcohol into the crucible, followed by heating the crucible or carrying out an ultrasonic treatment. After the solid encapsulant is dissolved, the boule is removed from the crucible by holding the crucible upside down, and either tapping the bottom portion of the crucible or deforming the crucible slightly.
A second known method is to hold a crucible, including a residual encapsulant and boule, upside down and to heat the crucible to a temperature above the softening point of the liquid encapsulant and then to remove the boule the same way as the first method.
However, in the known first method, a long time, for example, two or three days is required to permit removal of the boule, so the time efficiency is not improved. Another disadvantage of the first known method is that excessive exfoliation of an inner surface of the crucible can result after the residual liquid encapsulant and boule are removed. Such exfoliation not only reduces the number of times the crucible can be reused, but may cause residual liquid encapsulant to be caught on a portion of the side wall of the crucible which was peeled off by the exfoliations.
The problem of exfoliation is illustrated in FIGS. 1(a) and 1(b).
FIG. 1(a) shows a schematic sectional view of a crucible 12, which includes a liquid encapsulant 19 and a gallium arsenide (GaAs) boule 21. FIG. 1(b) shows a partial enlarged magnificent view of FIG. 1(a).
As is apparent from FIGS. 1(a) and 1(b), the more the crucible is used, the more difficult is removal of the boule from the crucible, because the exfoliations are increased. Accordingly, it is probable that the crucible will be damaged during removal of the boule.
In the second known method, the time for taking the boule out of the crucible is shorter than that of the first method, but a toxic gas is generated by decomposing a part of the boule, because the heating requires a temperature of about 800.degree. C. Accordingly, the removal operation must be performed in an isolation apparatus to provide for safety of the operator. Thus, the time efficiency for removing the boule from the crucible is not improved.